The enthalpy change for the oxidative deamination of glutamate by NADP ion catalyzed by bovine liver glutamate dehydrogenase has been determined calorimetrically. The delta H degrees values are 64.6 plus or minus 1.2 kJ/mol and 70.3 plus or minus 1.2 kJ/mol at 25 and 35 degrees C respectively. The equilibrium constants for the reaction at the two temperatures were determined spectrophotometrically. The thermodynamic parameters, delta G, delta H, and delta S, characterizing the tight binding of methotrexate, folates and pyridine nucleotides to chicken liver dihydrofolate reductase have been determined from calorimetric and fluorescence measurements. In addition, the enthalpy of methotrexate-enzyme interaction demonstrates a proton transfer associated with binding; this is not the case with folate and dihydrofolate, thus confirming the conclusions drawn from the observed difference spectra characteristic of the interaction of methotrexate and substrates with the enzyme. The thermodynamic parameters for the binding of NAD ion, NADH, ADP-ribose and 5-iodosalicylate to some selected dehydrogenases were determined calorimetrically at pH 7.6 and 25 degrees. This gives an indication of a possible extension of the structure-function correlation to the thermodynamics of binding coenzymes and coenzyme fragments as well.